1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a silicon single crystal having few crystal defects and a uniform oxygen concentration distribution, as well as to a silicon single crystal and silicon wafers manufactured by the same.
2. Description of the Related Art
Along with a decrease in size of semiconductor elements for coping with the increased degree of integration of semiconductor circuits, quality requirements are recently becoming severer on silicon single crystals which are grown by the Czochralski method (hereinafter referred to as the CZ method) for use as materials for substrates of semiconductor circuits. Particularly, there has been required a reduction in density and size of grown-in defects such as flow pattern defects (FPD), laser scattering tomography defects (LSTD), and crystal originated particles (COP).
In connection with the description of the above-mentioned defects incorporated into a silicon single crystal, first are described factors which determine the concentration of a point defect called a vacancy (hereinafter may be referred to as V) and the concentration of a point defect called interstitial (hereinafter may be referred to as I) silicon.
In a silicon single crystal, a V region refers to a region which contains a relatively large number of vacancies, i.e., depressions, pits, or the like caused by missing silicon atoms; and an I region refers to a region which contains a relatively large number of dislocations caused by excess silicon atoms or a relatively large number of clusters of excess silicon atoms. Further, between the V region and the I region there exists a neutral (hereinafter may be referred to as N) region which contains no or few missing or excess silicon atoms. Recent studies have revealed that the above-mentioned grown-in defects such as FPD, LSTD, and COP are generated only when vacancies and/or interstitials are present in a supersaturated state and that even when some atoms deviate from their ideal positions, they do not appear as a defect so long as vacancies and/or interstitials do not exceed the saturation level.
According to a popular view, in the CZ method, the concentration of vacancies and/or interstitials depends on the relation between the pulling rate of crystal and the temperature gradient G in the vicinity of a solid-liquid interface of a growing crystal (see FIG. 4), and another type of defect called oxidation-induced stacking fault (OSF) is present in the vicinity of the boundary between the V region and the I region (Erich Dornberger and Wilfred von Ammon, J. Electrochem. Soc., Vol. 143, No. 5, May 1996; T. Abe, H. Harada, J. Chikawa, Paper presented at ICDS-12 Amsterdam, Aug. 31-Sep. 3, 1982).
According to a conventional pulling method, in view of cost of growth and assuming that OSF does not exist in a V-rich region, pulling is mostly performed in the V-rich region, in which a crystal can be grown at a relatively high growth rate. Also, a thermal history or the like during pulling has been controlled so as to reduce crystal defects generated in the V-rich region. For example, according to control practiced in the conventional pulling method, a transit time across a temperature zone of 1150-1080xc2x0 C. is made relatively long so as to reduce the density of defects, each of which is conceivably a cluster of vacancies such as FPD, so that there can be improved the dielectric breakdown strength of oxide film, which is a factor for evaluating device characteristics. However, recent studies have revealed that such a method as controlling a thermal history (a transit time across a certain temperature zone) during pulling can reduce the density of defects, but the size of defects rather increases with a resultant failure to reduce a total volume of defects.
Thus, there has recently been made an attempt to reduce the pulling rate for quality improvement in spite of an increase in manufacturing cost or to increase a temperature gradient in the vicinity of the solid-liquid interface of a crystal as much as possible, to thereby manufacture a crystal that partially or entirely has an I-rich region, in which FPD, LSTD, COP, and like defects are observed less often. However, recent studies have revealed that even in the I-rich region, relatively large-sized Secco etch pit defects (hereinafter referred to as L-SEPD) are present at a portion located away from the boundary region between the V-rich region and the I-rich region. L-SEPD is conceivably a dislocation loop formed of a cluster of excess interstitial silicons. L-SEPD may be more likely to have an adverse effect on device characteristics than are FPD, LSTD, COP, and like defects generated in the V-rich region.
A recent tendency to increase the degree of integration of semiconductor devices requires uniformity of properties over a silicon wafer surface. Particularly, the oxygen concentration distribution is desired to be uniformly distributed over the surface of a wafer since the distribution directly influences the yield of devices.
Conceivable factors responsible for an impairment in the oxygen concentration distribution over the surface of a silicon wafer obtained from a crystal grown by the CZ method include convection of silicon melt, conditions of a gas atmosphere, rotation of a crystal, and rotation of a crucible. Particularly, since a cooling rate differs between an outer peripheral portion and an inner central portion of a growing crystal ingot, a crystal growth interface (a solid-liquid interface) does not become flat, thus having an adverse effect on the oxygen concentration distribution over the surface of a wafer.
Specifically, in the CZ method, at an inner portion of a crystal growth interface, crystal growth is relatively slow because of relatively slow cooling. As a result, the crystal growth interface becomes upwardly convex. A wafer obtained by slicing the thus-grown silicon bar has growth striations on the surface derived from different times of growth. Consequently, an oxygen concentration is distributed over the wafer surface in accordance with variations in oxygen concentration in the direction of crystal growth.
Conventionally, such variations and distribution of oxygen concentration derived from the profile of a solid-liquid interface are considered unavoidable in growing a single-crystal ingot by the CZ method.
Accordingly, taking for granted that variations and distribution of oxygen concentration derived from the profile of a solid-liquid interface are present to some extent, an attempt to improve the oxygen concentration distribution over a wafer surface has been carried out through the control of the above-mentioned factors such as rotation of a crystal.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the invention is to obtain at high productivity a silicon single crystal and a silicon wafer by the CZ method such that neither a V-rich region nor an I-rich region is present and a defect density is very low over the entire crystal cross section, as well as to improve the oxygen concentration distribution over the surface of a silicon wafer.
According to a first aspect of the present invention, there is provided a method for manufacturing a silicon single crystal in accordance with a Czochralski method, wherein during the growth of a silicon single crystal, pulling is performed such that a solid-liquid interface in the crystal, excluding a peripheral 5 mm-width portion, exists within a range of an average vertical position of the solid-liquid interface xc2x15 mm.
As a result of pulling a crystal such that the crystal growth interface (solid-liquid interface) in the crystal, excluding a peripheral 5 mm-width portion, exits within a range of an average vertical position of the solid-liquid interface xc2x15 mm, the crystal has only a neutral region (hereinafter referred to as the N region) and has neither a V-rich region nor an I-rich region, which contain many defects. Also, the oxygen concentration distribution over a wafer surface can be improved significantly. The peripheral 5 mm-width portion of the solid-liquid interface is excluded because the 5 mm-width portion exhibits significant variations in profile and is unstable.
According to a second aspect of the present invention, there is provided a method for manufacturing a silicon single crystal in accordance with the Czochralski method, wherein during the growth of a silicon single crystal, a furnace temperature is controlled such that a temperature gradient difference xcex94G (=Gexe2x88x92Gc) is not greater than 5xc2x0 C./cm, where Ge is a temperature gradient (xc2x0 C./cm) at a peripheral portion of the crystal, and Gc is a temperature gradient (xc2x0 C./cm) at a central portion of the crystal, both in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C. in the vicinity of the solid-liquid interface of the crystal.
Thus, during the growth of a crystal, through the adjustment of a so-called hot zone (hereinafter referred to as HZ), i.e., through the control of the furnace temperature such that the temperature gradient xcex94G (=Gexe2x88x92Gc) is not greater than 5xc2x0 C./cm-where Ge is the temperature gradient (xc2x0 C./cm) at the peripheral portion of the crystal, and Gc is the temperature gradient (xc2x0 C./cm) at the central portion of the crystal-pulling can be performed only in the N region between the V-rich region and the I-rich region. Also, the pulling rate can be determined accordingly. Thus, a silicon single crystal can be grown in a stable manner and at high productivity by the CZ method that is carried out while only the N region is formed so that the defect density is very low over the entire cross section of a crystal. Accordingly, wafer can be manufactured stably, while maintaining high productivity.
Also, during the growth of a crystal, through attainment of the temperature gradient difference xcex94G of not greater than 5xc2x0 C./cm, the solid-liquid interface in the crystal, excluding a peripheral 5 mm-width portion, exits within a range of an average vertical position of the solid-liquid interface xc2x15 mm. Thus, the oxygen concentration distribution over a wafer surface can be improved significantly.
In this case, the temperature gradient G (amount of change in temperature/length along crystal axis; xc2x0 C./cm) in the vicinity of the solid-liquid interface of a crystal may be a temperature gradient in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C., preferably between a melting point of silicon and 1400xc2x0 C. for execution of more accurate control.
According to a third aspect of the present invention, there is provided a method for manufacturing a silicon single crystal according to a Czochralski method with a magnetic field applied, wherein during the growth of a silicon single crystal, a furnace temperature is controlled such that a temperature gradient difference xcex94G (=Gexe2x88x92Gc) is not greater than 5xc2x0 C./cm, where Ge is a temperature gradient (xc2x0 C./cm) at a peripheral portion of the crystal, and Gc is a temperature gradient (xc2x0 C./cm) at a central portion of the crystal, both in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C. in the vicinity of the solid-liquid interface of the crystal.
Thus, in the Czochralski method with a magnetic field applied, through the pulling of a crystal such that xcex94G is not greater than 5xc2x0 C./cm, the N region expands, and thus the range of control expands accordingly. This further facilitates the growth of a silicon single crystal and a silicon wafer that are almost free of crystal defects.
Preferably, the applied magnetic field is a horizontal magnetic field. Also, preferably, the intensity of the applied magnetic field is not less than 2000 G.
In order to expand the N region and make the solid-liquid interface flat through the suppression of convection of a silicon melt, a horizontal magnetic field is preferred to a vertical magnetic field or to a cusp magnetic field. The application of a magnetic field does not yield much effect if the intensity of the magnetic field is less than 2000 G.
Further preferably, control is performed such that the length of a portion of a crystal corresponding to an in-crystal descending temperature zone between 1300xc2x0 C. and 1000xc2x0 C. is not longer than 8 cm. Still further preferably, control is performed such that a transit time across an in-crystal descending temperature zone of 1300-1000xc2x0 C. is not longer than 80 minutes.
The above-described conditions mean pulling conditions that determine the pulling rate or temperature gradient in a solid crystal portion located above the solid-liquid interface. Through the execution of the control under the conditions, the absolute value of the temperature gradient G becomes relatively large. Thus, even at a relatively high pulling rate, pulling can be performed in the N region.
When the length of a portion of a crystal corresponding to an in-crystal descending temperature zone between 1300xc2x0 C. and 1000xc2x0 C. is in excess of 8 cm, the absolute value of the temperature gradient G becomes relatively small. As a result, in order to obtain a silicon single crystal whose defect density is very low over the entire crystal cross section or a silicon wafer whose defect density is very low over the entire surface, the pulling rate must be made excessively low. Likewise, when a transit time across an in-crystal descending temperature zone of 1300-1000xc2x0 C. is longer than 80 minutes, the absolute value of the temperature gradient G becomes relatively small. As a result, in order to obtain a silicon single crystal whose defect density is very low over the entire crystal cross section or a silicon wafer whose defect density is very low over the entire surface, the pulling rate must be made excessively low. Thus, high productivity is difficult to maintain in a stable manner.
Still further preferably, the pulling rate and the temperature gradient G in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C. in the vicinity of the solid-liquid interface are adjusted such that the crystal is grown, across its entire cross section, in the neutral region near the boundary region between the vacancy-rich region and the interstitial-rich region. Thus, variation of defect density over the entire cross section of the crystal can be reduced.
When a crystal is pulled according to a conventional method which does not consider the concept of the N region, the temperature gradient difference xcex94G (=Gexe2x88x92Gc) becomes relatively large; consequently, formation of only the N region over the entire crystal cross section is impossible. However, as mentioned previously, through the control of xcex94G to a level not higher than 5xc2x0 C./cm and through the adequate adjustment of the pulling rate, the N region can be formed over the entire crystal cross section. The N region is little susceptible to the generations of vacancies or interstitials, establishes a relatively low defect density over the entire crystal cross section, and exhibits a relatively small variation in the defect density over the entire crystal cross section; thus, stable quality is imparted to a grown single crystal as well as wafers obtained from the single crystal.
According to a fourth aspect of the present invention, there is provided an apparatus for manufacturing a silicon single crystal according to a Czochralski method, wherein during the growth of a silicon single crystal, a furnace temperature is established such that a temperature gradient difference xcex94G (=Gexe2x88x92Gc) is not greater than 5xc2x0 C./cm, where Ge is a temperature gradient (xc2x0 C./cm) at a peripheral portion of the crystal, and Gc is a temperature gradient (xc2x0 C./cm) at a central portion of the crystal, both in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C. in the vicinity of the solid-liquid interface of the crystal. According to a fifth aspect of the present invention, there is provided an apparatus for manufacturing a silicon single crystal according to a Czochralski method with a magnetic field applied, wherein during the growth of a silicon single crystal, a furnace temperature is established such that a temperature gradient difference xcex94G (=Gexe2x88x92Gc) is not greater than 5xc2x0 C./cm, where Ge is a temperature gradient (xc2x0 C./cm) at a peripheral portion of the crystal, and Gc is a temperature gradient (xc2x0 C./cm) at a central portion of the crystal, both in an in-crystal descending temperature zone between 1420xc2x0 C. and 1350xc2x0 C. or between a melting point of silicon and 1400xc2x0 C. in the vicinity of the solid-liquid interface of the crystal.
In the apparatuses for manufacturing a silicon single crystal according to the present invention, a solid-liquid interface heat insulator is preferably arranged above a silicon melt so as to enclose a silicon single crystal, and a gap of 3-5 cm is formed between the surface of the melt and the bottom end of the solid-liquid interface heat insulator.
Through use of an apparatus capable of establishing xcex94G not higher than 5xc2x0 C./cm, particularly the apparatus with a magnetic field applied, a crystal can be pulled while only the crystal-defect-free N region is formed in the crystal. For example, through the arrangement of the solid-liquid interface heat insulator so as to enclose HZ and to form a gap of 3-5 cm between the surface of a melt and the bottom end of the solid-liquid interface heat insulator, the solid-liquid interface is sufficiently irradiated with radiant heat from a heater. Accordingly, the apparatus for manufacturing a silicon single crystal, which can control the temperature gradient difference xcex94G (=Gexe2x88x92Gc) in the vicinity of the solid-liquid interface to a level not higher than 5xc2x0 C./cm, can be obtained. Thus, there can be stably grown a silicon single crystal and a silicon wafer which contain very few crystal defects through use of the apparatus according to the present invention.
When the gap between the bottom end of the solid-liquid heat insulator and the surface of the melt is less than 3 cm, the irradiation of the solid-liquid interface with radiant heat from the heater or the like becomes insufficient. As a result, the temperature gradient difference xcex94G (=Gexe2x88x92Gc) in the vicinity of the solid-liquid interface attains a level in excess of 5xc2x0 C./cm. Thus, there cannot be formed the N region, which is neither the V-rich region nor the I-rich region contained many defects. As a result, there cannot be grown a silicon single crystal whose defect density is very low over the entire crystal cross section or a silicon wafer whose defect density is very low over the entire surface. By contrast, when the gap is in excess of 5 cm, the temperature gradient G becomes relatively small. As a result, in order to obtain a silicon single crystal whose defect density is very low over the entire crystal cross section or a silicon wafer whose defect density is very low over the entire surface, the pulling rate must be made excessively low. Thus, high productivity is difficult to maintain in a stable manner.
According to a sixth aspect of the present invention, there is provided a silicon single crystal grown by the method according to any of the first through third aspects or by the apparatus according to the fourth or fifth aspect.
Through the growth of a silicon single crystal by the method according to any of the first through third aspects or by the apparatus according to the fourth or fifth aspect, a silicon single crystal can be pulled stably in the N region. The thus-grown silicon single crystal contains very few crystal defects such as FPD, LSTD, COP, and L-SEPD, and a wafer obtained from the silicon single crystal exhibits an improved oxygen concentration distribution over the surface of the wafer.
According to an eighth aspect of the present invention, there is provided a silicon single crystal grown by the Czochralski method, wherein variation in oxygen concentration in a direction perpendicular to a growth direction is not higher than 5%.
According to the present invention, xcex94G is controlled to a level not higher than 5xc2x0 C./cm. Also, the profile of the solid-liquid interface is made sufficiently flat. Accordingly, there can be obtained a silicon single crystal having a uniform oxygen concentration distribution. Particularly, the present invention provides a single-crystal ingot which exhibits a uniform oxygen concentration distribution in a direction perpendicular to a growth direction over substantially the entire length of the bar (i.e., the oxygen concentration distribution is uniform over the entire surface of a wafer obtained through slicing the single-crystal ingot).
According to a ninth aspect of the present invention, there is provided a silicon wafer whose FPD density is not greater than 100 defects/cm2 and whose density of SEPD having a size not smaller than 10 xcexcm is not greater than 10 defects/cm2.
Thus, a silicon wafer obtained from a silicon single crystal that is grown according to the present invention contains very few grown-in defects such as FPD, LSTD, COP, and L-SEPD and thus is very useful.
Preferably, in a silicon wafer of the present invention, in addition to a feature of few crystal defects, an in-plane distribution of oxygen concentration is not greater than 5%.
Herein, the in-plane distribution of oxygen concentration is a value obtained by dividing the difference between the maximum value of oxygen concentration over the surface of a wafer and the minimum value of oxygen concentration over the surface of the wafer by the maximum value, or a value obtained by dividing the difference between the maximum value of oxygen concentration over the surface of the wafer and the minimum value of oxygen concentration over the surface of the wafer by the average value of oxygen concentration measurements. According to the present invention, the thus-calculated in-plane distribution of oxygen concentration becomes 5% or less.
The present invention reduces grown-in defects, such as FPD, L-SEPD, and COP, which are generated in a silicon single crystal grown by the CZ method or the MCZ method. The invention also enables a silicon single crystal to be grown such that a wafer obtained from the silicon single crystal is free of defects over the entire surface of the wafer. In the growth, productivity is hardly impaired by virtue of a relatively high pulling rate. Additionally, the variation of oxygen concentration over a wafer surface is improved.